We have previously characterized a DNA regulatory element in the second intron of the murine IL-4 gene. This element exhibits mast cell- specific enhancer activity in transient transfection assays. Analysis of the components of this element revealed that the cell specific activity is due to critical sites within the enhancer that serve as binding sites for transcription factors expressed in mast cells but no T cells. Although this element works well with a heterologous promoter in reporter gene assays, it has only marginal activity with the 5"IL-4 promoter, suggesting its physiological role is not involved in enhancing 5"IL-4 promoter-mediated transcription. It has recently become apparent that a TATA-less promoter element (a consensus Inr sequence) is located just downstream of the previously defined enhancer elements. The Inr- containing promoter element is located just downstream of the previously defined enhancer elements. The Inr-containing promoter element is active and appears to drive transcription of a unique, truncated IL-4 transcript. This transcript contains sequences that initiate immediately 3' of the Inr sequence that are spliced in exons II and IV. Studies of the expression profile of this "truncated" mRNA indicate it is expressed at relatively high levels in unstimulated mast cells and splenic cultures. This expression of this transcript is inversely correlated with that of the full length transcript: cell stimulation results in the disappearance of this mRNA species and an increase in the full length IL-4 element. Based on these data, we hypothesize that the truncated IL-4 mRNA is involved in the regulation of IL-4 production and/or activity, and its transcription is controlled by the intronic enhancer. This proposal describes experiments aimed at defining the function of the transcript and exploring the details of its expression and regulation.